feat: estimate cost for usage updates

[gilgm12]

Summary

• fetch OpenAI model pricing and attach estimated USD cost to ACP usage_update events
• support both new and resumed/loaded sessions, baselining resumed cumulative token totals so Superlite can add persisted prior cost without double-counting
• add focused tracker coverage for cumulative deltas and resume baselining

Verification

• npm exec vitest run src/tests/SessionCostTracker.test.ts src/tests/CodexACPAgent/token-usage-events.test.ts
• npm run typecheck
• npm run build

[gilgm12]

@benbrandt let me know if there's anything else you need from me on this.